Liquid crystal display devices are mounted on various apparatuses and devices such as a television receiver, an in-vehicle display, for example, car navigation equipment, a notebook computer, a tablet computer, mobile terminals, for example, a cell phone and a smartphone.
Such liquid crystal display devices employ liquid crystals whose modes vary in accordance with intended purposes.
For example, in a liquid crystal display device which is of a vertical-electric-field type such as a twisted nematic (TN) mode or an optically compensated (OCB) mode, the alignment direction of liquid crystal molecules contained in a liquid crystal layer held between a counter-electrode provided in an upper substrate and pixel electrodes provided in an lower substrate is controlled by an electric field generated between the counter-electrode and the pixel electrodes.
In a liquid crystal display device which is of a lateral-electric-field type such as an in-plane switching (IPS) mode or a fringe-field switching (FFS) mode, a counter-electrode (which is referred to as a common electrode in this type of liquid crystal device) and pixel electrodes are provided in a single substrate only, and the alignment direction of liquid crystal molecules contained in a liquid crystal layer is controlled by an electric field (fringe electric field) generated between the common electrode and the pixel electrodes. The FFS mode of liquid crystal display device can ensure a great aperture ratio, and thus has a high brightness and a superior viewing angle characteristic.
A liquid crystal display device for a mobile terminal is strongly required to reduce its circuit power consumption. As means for reducing the circuit power consumption, intermittent driving has been proposed. The intermittent driving is a method for reducing the circuit power consumption by providing a circuit stop period corresponding to a few display periods after writing is performed in a single display period. This method is not suitable for displaying of moving images, since the intervals at which an image signal is subjected to rewriting by a liquid-crystal display unit are long. However, in the case of displaying a still image, etc, i.e., in a display operation in which importance is not attached to visibility of moving images, the above method is considered as an advantageous countermeasure for reducing the circuit power consumption.
However, in the case where intermittent driving is performed in a liquid crystal display device, it is necessary to reduce flicker.
A time period in which an image signal of pixels is subjected to rewriting is referred to as a “frame period” or “single frame”; and the reciprocal thereof is also referred to as a “frame frequency”. It is assumed that those names can be applied with respect to the intermittent driving. In the case where the frame frequency was 60 Hz, which is adopted in an ordinary liquid crystal display device, flicker was not particularly noticeable. However, in the intermittent driving, in the case where the frame frequency was reduced, flicker was visually recognized. Also, in the case where the frame frequency was further reduced, flicker was particularly noticeable.